1. Field of the Invention
The present invention relates to a magnetic brush developer for use in developing an electrostatic latent image in electrophotography or the like.
2. Description of the Related Art
The process disclosed in U.S. Pat. No. 2,297,691 is known as an electrophotographic process. According to this process, in general, a uniform static charge is applied to a photoconductive insulator by corona discharge or the like, the insulator is then exposed imagewise to light by various means to form an electrostatic latent image. The latent image is then developed and visualized by a fine powder called "a toner". The toner image is transferred onto a paper sheet or the like according to need, and the transferred toner image is then fixed by compression, application of heat, a solvent vapor or light to obtain a print.
As the toner for developing an electrostatic latent image, particles obtained by pulverizing a dispersion of a colorant such as carbon black in a binder resin composed of a natural or synthetic polymeric substance of about 1 to 30 .mu.m have been used. Generally, the toner is mixed with a carrier such as iron powder to form a magnetic brush developer, and this developer is used for developing an electrostatic latent image.
The process for developing the electrostatic latent image is roughly divided into two methods. The first is a positive development method in which toner particles having a reverse polarity to that of a photoconductive insulator (photoconductor) adhere to the static charge-remaining region on the photoconductor. The second is a reversal development method in which toner particles having the same polarity as that of the photoconductor adhere to the static charge-free region. In the reversal development method, a direct current voltage (bias voltage) having the same polarity as that of the latent image is applied to a magnetic roll (sleeve) to effect the transfer of the developer. In conventional copying machines, the positive development method is mainly used, but where the positive development method is used in a laser printer, the printing ratio is ordinarily a few %, and it is therefore necessary to irradiate a major portion of the photoconductor with light to erase the static charge. Problems arise in connection with the short life of the laser and the precision of the optical system. Accordingly, the reversal development method is often used in conventional laser printers.
The problem in the reversal development process is that the toner adheres to the sleeve while development is repeated. If this adhesion occurs, the sleeve becomes an insulator and it becomes impossible to apply the development bias voltage, with the result that a sharp and clearn image cannot be obtained. This phenomenon occurs because the toner is attracted to the sleeve by the electrostatic repulsive force generated when the polarity of the toner is the same as that of the static charge, and by the electric lines of force generated according to the voltage difference between the photoconductor (high voltage) and the sleeve (low voltage). This phenomenon occurs frequently when the gap between the photoconductor and the sleeve is narrow.
Contributions of the constituent materials of the developer to the development will now be described. An important role of the carrier is to give an appropriate charge to the toner. Since this charging is caused by electrostatic friction between the toner and carrier, setting of the tribo-electric series for the toner and carrier is important. If the developer is used for a long time, so-called toner filming, that is, adhesion of the toner to the surface of the carrier, results this changes the charging characteristics of the carrier with the result that it becomes impossible to impart a sufficient charge to the toner and the print quality is therefore degraded. If the toner charge is reduced simultaneously with or before this degradation, the toner will be apt to separate from the carrier and a toner coating readily forms on the sleeve. The adhesion of toner to the sleeve is therefore caused by repetition of the development. To eliminate this disadvantage, the reduction of the tribo-electric property in continuous printing must be prevented by appropriate control of the tribo-electric coordinates (positions in tribo-electric series) for the toner and carrier. For this purpose, it is necessary to coat the surfaces of carrier particles with a resin which is non-sticky to the toner.
To prevent adhesion of the toner to the sleeve, control of the tribo-electric coodinates for the toner and carrier is especially important. As a means for imparting positive chargeability or negative chargeability, a method has been adopted in which a positive charge control agent or negative charge control agent is added to the toner. However, if this method is adopted, the self chargeability of the toner per se is increased and the toner is readily attracted by an electric field directed to the sleeve from the photoconductor, therefore a coating of the toner readily forms on the sleeve. As pointed out above, adhesion of the toner to the sleeve is a serious problem for a printer in which the reversal development process is adopted. This problem must be solved by improving the developer.
In the two-component type magnetic brush developer previously mentioned, the problem of toner adhesion to the surface of the carrier is generally caused by physical contact between the carrier and toner. Therefore, if the tribo-electric property of the toner changes, the electric resistance of the carrier changes and the image quality becomes degraded. For example, development of solid areas becomes impossible. Accordingly, it is desirable to provide a developer in which the tribo-electric property and the electric conductivity of the carrier changes little or not at all, even after continuous printing.
Another problem concerning the toner for electrophotography resides in the fixation process. The fixation process involves melting the toner powder image and fixing the toner image to a paper. There are various fixing methods as described above, but in conventional copying machines and printers, fixation by hot roll is commonly adopted. In hot roll fixation the toner is generally a binder resin comprising a low-molecular-weight component and a high-molecular-weight component is used. More specifically, a sufficient fixing quality is obtained by the low-molecular-weight component and offsetting of the hot roll is prevented by the high-molecular-weight component. It is considered that offsetting is a cohesive failure which occurs when the adhesive force between the toner and the hot roll is greater than the cohesive force of the toner. Accordingly, to prevent offsetting, a wax must be added to reduce the adhesive force between the toner and the hot roll or rather, a stronger cohesive force must exist in the polymer molecules of the molten toner relative to the adhesive force of the same. Polypropylene or montanic acid wax is generally used. However, use of a wax is not preferred because the flowability of the toner is degraded such that toner filming of the photoconductor causes an increase in the background image. Although a method is often adopted in which the ratio of the high-molecular-weight component in the binder resin is increased to improve the cohesive force of the toner, this method is not preferred because toner fixing quality is degraded. Accordingly, a binder resin capable of imparting a good fixing property and an excellent offset-preventing property is desired.